Fix root cause: naive vs aware datetime crash + stuck AI mode indicator

The autonomous loop was crashing on every decision with:
  TypeError: can't subtract offset-naive and offset-aware datetimes
because MongoDB stores created_at without timezone info but the code
compared it against datetime.now(timezone.utc).

- conversation_context_service: make created_at timezone-aware before
  subtraction (replace tzinfo=utc when naive)
- DiscussionPanel: fix sync effect — when server reports AI mode is
  inactive, always clear localAiModeActive regardless of its value,
  so the "AI is generating..." spinner doesn't get stuck when the
  backend fails/stops before the frontend has confirmed AI mode started

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/conversation_context_service.py

@@ -50,6 +50,9 @@ class ConversationContextService:
             # Calculate elapsed time
             created_at = focus_group.get('created_at')
             if created_at:
+                # MongoDB may return naive datetimes — make timezone-aware before subtracting
+                if created_at.tzinfo is None:
+                    created_at = created_at.replace(tzinfo=timezone.utc)
                 elapsed_minutes = (datetime.now(timezone.utc) - created_at).total_seconds() / 60
             else:
                 elapsed_minutes = 0

src/components/focus-group-session/DiscussionPanel.tsx

@@ -77,12 +77,13 @@ const DiscussionPanel = ({
   // Calculate reasoning panel visibility - only show when user explicitly expands it
   const reasoningPanelVisible = reasoningPanelExpanded;
 
-  // Sync localAiModeActive back to null once parent prop has caught up
+  // When server reports AI is NOT active, clear any optimistic local override immediately.
+  // This handles the case where AI mode fails/stops before the frontend confirms it started.
   useEffect(() => {
-    if (localAiModeActive !== null && localAiModeActive === isAiModeActive) {
+    if (!isAiModeActive && localAiModeActive !== null) {
       setLocalAiModeActive(null);
     }
-  }, [isAiModeActive, localAiModeActive]);
+  }, [isAiModeActive]);
 
   // Fetch reasoning history when in AI mode
   useEffect(() => {